Various types of electrical signal filters are used in the CATV industry for controlling, on a frequency basis, the propagation of signals through a cable line. One example of such a filter is known as a notch filter. It is important that such notch filters offer a high level of attenuation, as well as precise and easy tuning capabilities, while maintaining a small size and economical construction.
A high level of attenuation can be realized by using a plurality of interconnected filter circuits on one or more circuit boards within the notch filter assembly. However, in this situation, it is critical that the multiple filter circuits (i.e., filter sections) be magnetically isolated from one another to avoid interference, such as cross-talk or magnetic coupling, between the filter circuits within the filter housing.
Using isolation shields to prevent unwanted cross-talk between filter circuits within a filter is known. One example of a filter having multiple filter circuits and including isolation shielding is disclosed in U.S. Pat. No. 4,451,803, the entirety of which is incorporated herein by reference. The ""803 patent discloses a split tuning notch filter for removing a selected frequency or band of frequencies from a CATV signal. With reference to FIG. 8, a split tuning filter includes a common circuit board 100 having first 102 and second 103 filter sections formed thereon by discrete electronic components such as inductors, capacitors and the like (not shown).
Isolation shields 104, 105 are arranged at a midpoint along circuit board 100 to provide magnetic isolation between first filter section 102 and second filter section 103. Each shield includes a radially extending disc section 106 and a longitudinally extending flange section 107. A slot 108 is formed in each shield, to allow the remaining, unslotted portion of disc 106 to slide into a corresponding slot 101 formed in circuit board 100.
One of the shields is positioned in a slot formed on one side of the circuit board, and the other shield is positioned in a slot formed on an opposed side of the circuit board, as shown in FIG. 8. As explained in the ""803 patent, this type of arrangement prevents any xe2x80x9cline of sightxe2x80x9d communication between components in the first and second filter sections. Once the shields 104, 105 are positioned on opposite sides of circuit board 100 and soldered in place. The circuit board is inserted into housing 109, and the shields are then soldered into the housing 109. The open end of the housing 109 is then closed by assembling the filter cap 110. This subassembly is then often inserted into a tube sleeve housing (not shown) to form the final filter structure.
Another example of a filter having multiple isolated filter sections is disclosed in U.S. Pat. No. 5,150,087. Like the ""803 patent, the ""087 patent uses a pair of manually laterally inserted, axially opposed isolation shields to separate multiple filter sections. However, unlike the single circuit board used in the ""803 patent, the ""087 patent uses a plurality of isolated independent circuit boards interconnected by a wire through the pair of shields. Nonetheless, in order to achieve the proper isolation and grounding, two shields are required to prevent line of sight between the two circuit boards. But even a single circuit board having multiple filter circuits (e.g., ""803 patent) typically requires at least two axially opposed isolation shields to accommodate a conductor or conductive trace (interconnecting multiple filter sections) while otherwise magnetically isolating the filter sections and preventing a line of sight therebetween. If the conductive trace is printed on the circuit board, it is also necessary for the slot 108 in each shield to include a clearance to prevent contact with the conductive trace.
While filters, such as the ones disclosed in the ""803 and ""087 patents, can successfully provide magnetic isolation between the first and second filter sections, there are several drawbacks associated with the use of such shield pairings. For example, although the discrete electrical components can be assembled on a circuit board using automated Z-axis manufacturing techniques and then wave soldered onto the circuit board en mass in a single economical and efficient manufacturing step, subsequent assembly steps, i.e., shield assembly and soldering steps, require substantial, precise manual labor.
More specifically, the shields must be manually attached to the circuit board by laterally positioning and fixturing the two shields into the corresponding slots in the circuit board. The shields must then be soldered to the circuit board before insertion into the housing. After insertion into the filter housing, the shields must again be soldered to the filter housing in order to properly ground the shields and the circuit board. The amount of manual assembly and soldering required in such a manufacturing process drives up the production cost and, in turn, increases the final cost to customers.
Thus, an electronic filter assembly, including a single circuit board separated into distinct and isolated filter sections using isolation shields, that can be economically produced using an automated manufacturing process, involving few, if any, manual assembly steps is desired. An electronic signal filter having a single circuit board including multiple filter circuits separated by isolation shields that can be automatically assembled onto the circuit board using Z-axis robotics-type automated assembly is also desired. Further, a substantially automated method of manufacturing such filters is desired, and it is especially desired that the automation steps be efficiently performed in a Z-axis direction with respect to an X-Y plane in which the circuit board resides.
It is an object of the present invention to overcome the drawbacks of the prior art. More particularly, it is an object of the present invention to provide an electronic signal filter having a single circuit board including multiple filter circuits separated by isolation shields that can be automatically, and economically, assembled onto the circuit board using Z-axis robotics-type automated assembly performed in a Z-axis direction with respect to an X-Y plane in which the circuit board resides.
According to a first embodiment of the present invention, an electronic signal filter is provided including a cylindrical housing adapted to be electrically grounded, and having a first end, an opposed second end and an inner peripheral surface defining an interior compartment. The electronic signal filter also includes a single circuit board positioned within the interior compartment of the cylindrical housing, the single circuit board having a first surface, an opposed second surface, a first filter section proximate the first end of the cylindrical housing and a second filter section proximate the second end of the cylindrical housing. The circuit board is positioned such that it effectively divides the interior compartment into a first compartment defined by the first surface of the circuit board and a first portion of the inner peripheral surface of the cylindrical housing, and a second compartment defined by the second surface of the circuit board and a second portion of the inner peripheral surface of the cylindrical housing. As explained below in further detail, it is preferred that the circuit board is positioned at a location below the centerline of the filter housing.
A first shield member is also provided, extending from the first surface of the circuit board toward the first inner peripheral surface of the cylindrical housing. The electronic signal filter further includes a second shield member radially opposing the first shield member extending from the second surface of the single circuit board toward the second inner peripheral surface of the cylindrical housing, the second shield member being electrically connected the first shield member, and the second shield member being a discrete component with respect to the first shield member.
Preferably, the first shield member includes a first portion extending from the first surface of the circuit board toward the first inner peripheral surface of the cylindrical housing and an integral second portion extending into the circuit board. Further, the second shield member includes a first portion extending from the second surface of the single circuit board toward the second inner peripheral surface of the cylindrical housing, and an integral second portion extending into the circuit board.
More preferably, the second portion of the first shield member is received within a slot in the circuit board, and preferably passes through the circuit board into the second compartment of the cylindrical housing. The second portion of the first shield member also preferably includes a securing member to mechanically couple the first shield member to the circuit board proximate the second surface of the circuit board. In that manner, the first shield member can be placed on the circuit board using Z-axis robotics type manufacturing techniques, and once positioned, the securing member is engaged to mechanically couple the first shield member to the circuit board.
This mechanical connection provides stability throughout the remainder of the pre-soldering assembly process. As mentioned below in further detail, the first shield member can be thusly secured onto the circuit board either before or after the remainder of the discrete filter components are placed in appropriate positions on the circuit board, or contemporaneously therewith. However, since the minimal amount of Z-axis force needed to engage the securing member could potentially disturb other loosely fit or otherwise unaffixedly positioned filter components on the conveyor, it is preferred that the first shield member be secured onto the circuit board before the additional components are placed thereon. And although it is still preferred that the first shield member be soldered onto the circuit board, this can be accomplished by mass wave soldering after all of the discrete filter components, including the first shield member, have been assembled onto the circuit board.
It is preferred that the second portion of the first shield member includes a spacer member extending in a direction parallel to a plane of the circuit board (i.e., an X-axis direction) to maintain an axial clearance between a surface of the second portion of the first shield member and an opposed edge of the slot formed in the circuit board. This spacer member adds stability to the connection between the first shield member and the circuit board, and aids in preventing unwanted lateral movement in the X-axis direction of the plane of the circuit board. The axial clearances occupy a portion of the slot in the circuit board opposing the spacer member over a distance in the Y-axis direction. In that manner, once positioned, the second portion of the second shield member extends through the circuit board within the axial clearance. The positioning of the lower shield member in that way is explained in further detail below.
It is also preferred that one of the first and second surfaces of the circuit board includes a conductor path (e.g., conductive trace) electrically connecting the first filter section and the second filter section, and a respective one of the first shield member and the second shield member includes a section positioned adjacent the conductor path that is spaced a distance from the conductor path to prevent contact therebetween. More preferably, the distance between the conductor path and the section of a respective one of the first and the second shield members is dimensioned to provide a spark gap. That is, the dimension of the space is selected to shunt current passing through the conductive trace to the grounded shield in the event of an unacceptably high voltage surge passing through the filter.
According to another embodiment of the present invention, the first shield member comprises a first plate having a first portion extending from the first surface of the circuit board toward the first inner peripheral surface of the cylindrical housing and an integral second portion extending into the circuit board, and a second plate axially spaced from the first plate and having a first portion extending from the first surface of the circuit board toward the first inner peripheral surface of the cylindrical housing and an integral second portion extending into the circuit board. A connection member is also provided, connecting the first plate and the second plate proximate the outer periphery of the first portions thereof and contacting the first inner peripheral surface of the cylindrical housing once inserted therein. A second shield member is positioned radially opposing the first shield member and also has a first plate having a first portion extending from the second surface of the circuit board toward the second inner peripheral surface of the cylindrical housing and an integral second portion extending into the circuit board, and a second plate axially spaced from the first plate and having a first portion extending from the second surface of the circuit board toward the second inner peripheral surface of the cylindrical housing and an integral second portion extending into the circuit board. A connection member is also included in the second shield member, connecting the first plate and the second plate proximate the outer periphery of the first portions thereof and contacting the second inner peripheral surface of the cylindrical housing once inserted therein.
Preferably, the second portions of the first and the second plates of the first shield member pass through the circuit board into the second compartment of the cylindrical housing, and at least one of the second portions of the first and the second plates includes a securing member to mechanically couple the first shield member to the circuit board proximate the second surface of the circuit board. Again, the placement of the first shield member can be achieved using Z-axis automation techniques, and once engaged, the securing member holds the first shield member in place on the circuit board for the duration of the pre-soldering assembly process.
The second portions of the first and the second plates of the first shield member are preferably received within a slot in the circuit board, and at least one of the second portions preferably includes a spacer member extending in a direction parallel to a plane of the circuit board to maintain an axial clearance between a respective surface of the second portion and an edge of the slot formed in the circuit board. More preferably, the second portions of the first and the second plates of the second shield member extend through the circuit board within the axial clearance. The second portions of the first and second plates of the second shield member are thusly press-fit into the axial clearances to provide a completed shield assembly, and the stability of the connection is enhanced by a soldering step that can be performed before and after the circuit board is inserted into the filter housing.
It is preferred that one of the first and second surfaces of the circuit board includes a conductor path printed thereon, electrically connecting the first filter section and the second filter section, and at least one of the first and the second plates of a respective one of the first and the second shield members comprises a section positioned adjacent the conductor path that is spaced a distance from the conductor path to prevent contact therebetween. It is also preferred that the first portion of the shield extending from the surface of the circuit board opposite the printed surface thereof be greater than half of the total inner area of the cylindrical filter housing in order to better accommodate taller discrete filter components assembled thereon. In this case, the circuit board would be positioned in the filter housing below the centerline thereof.
Preferably, the distance between the conductor path and the section of at least one of the first and the second plates is dimensioned to provide a spark gap. That is, the dimension of the space is selected to shunt current passing through the conductive trace to the grounded shield in the event of an unacceptably high voltage surge passing through the filter. In this case, it is only necessary to provide such spark gap protection proximate the surface of the circuit board having the conductor path, and the opposing shield member can be positioned to be flush with respect to the non-printed surface of the circuit board. It is also possible, however, to include a conductive via in electrical communication with the conductor path, positioned proximate the section of the respective shield plate on the other surface of the circuit board, that passes through the circuit board to the other surface thereof. When this via is provided, it is also preferred to provide a corresponding section dimensioned on the respective shield member directly opposing the via as a secondary spark gap.
Accordingly, when each section of the first and second plates of both the first and second shield members are dimensioned to shunt current passing through the conductor path to the grounded shield in the event of a voltage surge passing through the filter, and when two vias are provided therewith, four spark gap points are offered. In addition to providing four points of protection, this precautionary measure increases the overall number of gaps and decreases the chances that all of the gaps will be rendered ineffective if and when the filter housing is filled with a stabilizing material.
According to yet another embodiment of the present invention, a method of manufacturing an electrical filter including an isolation shield assembly is provided. The method includes the steps of:
a. providing at least one circuit board having a first surface and a second surface;
b. positioning a plurality of discrete filter components on the first surface of the circuit board, forming a first filter section and a second filter section;
c. positioning a first shield member on the first and second surface of the circuit board interposed between the first and the second filter sections;
d. simultaneously soldering the discrete filter components and the first shield member in place on the circuit board;
e. positioning a second shield member on the second surface of the circuit board; and
f. positioning the circuit board with the shields and the filter components within a filter housing.
According to the method of the present invention, step b can be performed before or after step c. However, it is preferred that step b and step c are performed substantially simultaneously (i.e., within a single boarding operation). This is because, as mentioned above, the force required to engage the securing members of the first shield members can jar or otherwise disturb unsecured discrete filter components already positioned on the circuit board. But when all of the filter components, including the first shield members, are substantially simultaneously positioned on the circuit board using Z-axis manufacturing techniques, this effect is not experienced and manufacturing efficiency is increased.
According to yet another embodiment of the method of the present invention, a step of soldering the second shield member is performed between step e and step f, and another step of soldering at least one of the first and the second shield members within the filter housing is performed after step f.
All of the embodiments of the present invention beneficially enable the use of Z-axis automation techniques in the manufacture thereof, which techniques are not feasible with respect to the prior art electronic signal filters that use disc-shaped shield members, as mentioned above. Accordingly, the present invention offers an estimated savings in manufacturing costs from about 10%-15%.